Briaexcavatolides K−N, New Briarane Diterpenes from the Gorgonian

Feb 20, 2001 - Four new briarane diterpenes, briaexcavatolides K−N (1−4), along with a known ... Journal of Natural Products 2002 65 (12), 1904-19...
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318

J. Nat. Prod. 2001, 64, 318-323

Briaexcavatolides K-N, New Briarane Diterpenes from the Gorgonian Briareum excavatum Ping-Jyun Sung,† Jui-Hsin Su,† Chang-Yih Duh,† Michael Y. Chiang,‡ and Jyh-Horng Sheu*,† Department of Marine Resources and Department of Chemistry, National Sun Yat-Sen University, Kaohsiung, 804, Taiwan, Republic of China Received June 2, 2000

Four new briarane diterpenes, briaexcavatolides K-N (1-4), along with a known diterpene, 5, have been isolated from the Taiwanese gorgonian Briareum excavatum. The structures of the new metabolites were established by extensive spectral analyses. Furthermore, the structure, including the relative configuration of briaexcavatolide K (1), was confirmed by a single-crystal X-ray analysis. Briaexcavatolides K and L (1 and 2) are the only briarane diterpenes known to possess hydroxyl groups at the C-8β and C-17R positions, respectively. Cytotoxicity of these metabolites toward various cancer cell lines also is described. In the past 24 years, many diterpenes possessing the briarane carbon skeleton, which contain a γ-lactone in a bicyclo[8.4.0] system, have been isolated from marine coelenterates mainly from the subclass Octocorallia. The briaranes raised interest not only due to their novel structures but also for their extensive biological activities.1 In a continuing survey of Taiwanese marine organisms with promising bioactive substances, the gorgonian Briareum excavatum (Nutting) (phylum Cnidaria, order Gorgonacea, family Briareidae) has been investigated. Previous studies on the chemical constituents of B. excavatum had led to the isolation of 36 briarane derivatives, excavatolides A-Z1-4 and briaexcavatolides A-J.5 In this paper, we report the isolation, structure determination, and cytotoxicity of four new briarane diterpenes, briaexcavatolides K-N (1-4), together with a known diterpene, (1S*,2S*, 5Z,7S*,8S*,9S*,10S*,11R*,12R*,13Z,17R*)-2,12-diacetoxy8,17-epoxy-9-hydroxybriara-5,13-dien-18-one (5)6 (asterisk denotes relative configuration), from the gorgonian B. excavatum. The structures of these compounds were established by spectroscopic methods. The relative configuration of briaexcavatolide K (1) was determined by a singlecrystal X-ray analysis. Results and Discussion Specimens of B. excavatum were frozen immediately after collection and subsequently freeze-dried. The freezedried organism was extracted with EtOAc to afford a crude extract. The extract was separated by column chromatography on silica gel and afforded diterpenes 1-5. Briaexcavatolide K (1) was isolated as colorless prisms, and a molecular formula C28H40O13 was established by HRFABMS. Accordingly, nine degrees of unsaturation were determined for the molecule of 1. The IR absorptions of 1 showed the presence of hydroxyl (νmax 3428 cm-1), γ-lactone (νmax 1772 cm-1), and ester carbonyl (νmax 1734 cm-1) groups in 1. The FABMS of 1 exhibited peaks at m/z 585 [M + H]+, 567 [M + H - H2O]+, 525 [M + H - HOAc]+, 507 [M + H - H2O - HOAc]+, 465 [M + H - 2HOAc]+, 447 [M + H - H2O - 2HOAc]+, 405 [M + H - 3HOAc]+, 345 [M + H - 4HOAc]+, 327 [M + H - H2O - 4HOAc]+, 309 [M + H - 2H2O - 4HOAc]+, and 291 [M + H - 3H2O * To whom correspondence should be addressed. Tel: 886-7-5252000 ext. 5030. Fax: 886-7-5255020. E-mail: [email protected]. † Department of Marine Resources, National Sun Yat-Sen University. ‡ Department of Chemistry, National Sun Yat-Sen University.

10.1021/np000287d CCC: $20.00

- 4HOAc]+, also suggesting the presence of three hydroxyl and four acetoxyl groups in the molecule of 1. From the 13C NMR spectral data of 1 (Table 2), a trisubstituted olefin was deduced from the signals of two carbons at δ 141.9 (s) and 125.1 (d). Furthermore, in the 13C NMR spectrum of 1, five carbonyl resonances appear at δ 176.1 (s), 170.7 (s), 170.6 (s), 170.1 (s), and 169.4 (s), confirming the presence

© 2001 American Chemical Society and American Society of Pharmacognosy Published on Web 02/20/2001

Briaexcavatolides K-N from Briareum

Journal of Natural Products, 2001, Vol. 64, No. 3 319

Table 1. 1H Chemical Shifts for Diterpenes 1-4 1a 2 3 4 4β R 6 7 9 10 11 12 13β R 14 15 16 17 18 20 acetate methyls

2b

5.83 br s 4.88 br s

5.83 br s 4.89 br s

3.21 dd (15.6; 4.2)c 2.02 br d (15.6) 5.55 d (9.6) 5.40 d (9.6) 6.10 br s 2.99 d (6.4) 2.16 m 3.88 br s 1.98 m 1.90 m 5.24 dd (10.8; 6.4) 1.53 s 1.79 s

3.21 dd (15.3; 3.9) 2.02 br d (15.3) 5.55 d (8.7) 5.40 d (8.7) 6.09 br s 2.98 d (6.6) 2.14 m 3.88 br s 1.99 m 1.91 m 5.24 dd (10.8, 6.3) 1.52 s 1.78 s

1.26 s 1.09 d (7.2) 2.20 s 2.13 s 2.02 s 1.95 s

1.25 s 1.08 d (7.2) 2.22 s 2.11 s 1.94 s

n-butyrate

3b

4b

4.18 d (9.9) 5.87 br t (10.8) 6.22 d (10.8)

4.13 d (9.9) 5.82 br t (10.5) 6.27 d (10.5)

5.81 d (8.7) 5.08 d (8.7) 5.14 d (6.4) 1.94 m 2.00 m 4.81 d (4.8) 3.23 br s

5.75 d (8.7) 5.20 d (8.7) 5.16 d (6.9) 1.95 m 2.04 m 4.75 d (4.8) 3.23 br s

3.23 br s 1.10 s 4.21 br s 2.33 q (6.9) 1.10 q (6.9) 1.04 d (6.9) 2.19 s 2.13 s

3.23 br s 1.14 s 4.30 br s 2.32 q (7.2) 1.16 q (7.2) 1.06 d (7.2) 2.18 s 2.12 s

CH2 2.26 t (7.5) CH2 1.66 m CH3 0.95 t (7.5)

a Spectra recorded at 400 MHz in CDCl . b 300 MHz in CDCl . c J values (in Hz) in parentheses. The values are ppm downfield from 3 3 TMS.

Table 2.

13C

Chemical Shifts for Diterpenes 1-4 1a

42.7 (s)c 72.8 (d) 73.4 (d) 32.9 (t) 141.9 (s) 125.1 (d) 85.4 (d) 81.1 (s) 65.1 (d) 43.8 (d) 37.0 (d) 69.8 (d) 34.5 (t) 74.3 (d) 20.0 (q) 22.7 (q) 76.9 (s) 16.6 (q) 176.1 (s) 15.5 (q) 22.0 (q) 21.0 (q) 21.0 (q) 20.8 (q) acetate carbonyls 170.7 (s) 170.6 (s) 170.1 (s) 169.4 (s) n-butyrate

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 acetate methyls

2b

3b

4b

40.3 (s) 75.8 (d) 136.8 (d) 126.2 (d) 140.8 (s) 124.8 (d) 79.3 (d) 81.8 (s) 70.0 (d) 36.8 (d) 38.0 (d) 71.9 (d) 57.3 (d) 62.4 (d) 15.0 (q) 46.5 (t) 43.2 (d) 6.3 (q) 175.7 (s) 9.7 (q) 21.8 (q) 21.0 (q)

40.4 (s) 75.6 (d) 136.0 (d) 125.1 (d) 145.4 (s) 120.0 (d) 79.9 (d) 81.5 (s) 70.0 (d) 37.2 (d) 37.7 (d) 72.0 (d) 57.8 (d) 63.1 (d) 15.0 (q) 63.8 (t) 43.5 (d) 6.4 (q) 177.1 (s) 9.6 (q) 21.8 (q) 21.1 (q)

171.0 (s) 170.5 (s) 170.4 (s) 169.8 (s) 169.3 (s)

171.1 (s) 170.1 (s)

42.8 (s) 72.9 (d) 73.2 (d) 33.0 (t) 141.7 (s) 125.2 (d) 85.5 (d) 81.2 (s) 65.2 (d) 43.8 (d) 37.0 (d) 69.6 (d) 34.6 (t) 74.5 (d) 19.9 (q) 22.6 (q) 76.8 (s) 16.2 (q) 176.7 (s) 15.5 (q) 21.8 (q) 21.0 (q) 21.0 (q)

173.2 (s) CH3 13.6 (q) CH2 17.9 (t) CH2 35.8 (t)

a Spectra recorded at 100 MHz in CDCl . b 75 MHz in CDCl . 3 3 Multiplicity deduced by DEPT and indicated by usual symbols. The values are ppm downfield from TMS.

c

of a γ-lactone and four other ester groups. In the 1H NMR spectrum of 1 (Table 1), signals for four acetate methyls are observed at δ 2.20 (3H, s), 2.13 (3H, s), 2.02 (3H, s), and 1.95 (3H, s). On the basis of the above data, diterpene 1 was found to be a tricyclic compound. The structure and all of the 1H and 13C chemical shifts of 1 were determined

by the assistance of 2D NMR studies, including an HMBC experiment (Table 3). Four acetoxyl groups were shown to be attached at C-2, C-3, C-9, and C-14 positions, respectively, based on the observed HMBC correlations. Thus, the remaining hydroxyl groups had to be attached at C-8, C-12, and C-17 positions. The relative stereochemistry of 1 was deduced from a NOESY experiment (Figure 1 and Table 4). The NOE correlations of H-10 with H-3 and H-11 indicated that these protons are situated on the same face and were assigned as the R protons since the C-15 methyl is β-oriented and H3-15 did not show a correlation with H-10. H-14 was found to exhibit NOE responses with H-13β and H3-15, but not with H-10, revealing the β-orientation of this proton. Also, the hydroxyl group at C-12 was found to be in the β face by the NOE correlations between H-12 with H-11 and H-13R. H-7 showed NOE correlations with H-4β and H315, confirming the β-orientation for this proton. H-9 was found to exhibit correlation with H3-20. From consideration of molecular models, H-9 was found to be reasonably close to H3-20 when H-9 was R-oriented and H3-20 was placed on the β face. The acetoxyl group at the β face of C-2 was confirmed by the NOE responses between H-2 with H-3 and H-13R. However, the stereochemistry of hydroxyl groups at the C-8 and C-17 positions cannot be fully determined by this way. A single-crystal X-ray structure analysis was carried out in order to confirm the molecular structure of 1. The X-ray structure (Figure 2) demonstrates the locations of two hydroxyl groups on the β-orientation of C-8 and the R-orientation of C-17, respectively. The structure, including the relative stereochemistry of diterpene 1, was thus elucidated unambiguously. Briaexcavatolide L (2) was isolated as a white powder and had the molecular formula C30H44O13 on the basis of EIMS and 13C NMR spectral data. Carbonyl resonances in the 13C NMR spectrum of 2 (Table 2) at δ 176.7 (s), 173.2 (s), 171.0 (s), 170.4 (s), and 169.3 (s) revealed the presence of a γ-lactone and four other esters in 2. In the 1H NMR

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Table 3. HMBC Correlations for Diterpenes 1-4 carbon 1 2 3 4 5 6 7 8 9 10 11 12 14 15 16 17 18 19 20 2-OCOMe 3-OCOMe 9-OCOMe 12-OCOMe 14-OCOMe 3-OCOPr

1

2

H-2, H-10, H-12, H-14, H3-15 H-10, H3-15 H-2 H-2, H-6, H3-16 H2-4, H-7, H3-16 H-7, H3-16 H-6 H-7, H-9,

H-2, H-10, H-12, H-14, H3-15 H-10, H3-15 H-2 H-2, H-6, H3-16 H2-4, H-7, H3-16 H-7, H3-16 H-6 H-7, H-9,

H-9, H-11, H-12, H3-15, H3-20 H-9, H-10, H3-20 H-10, H3-20 H-2, H-10, H-12, H2-13, H3-15

H-9, H-11, H-12, H3-15, H3-20 H-9, H-10, H3-20 H-10, H3-20 H-2, H-10, H-12, H2-13, H3-15

H-6 H-9, H3-18

H-6 H-9, H3-18

H-7, H3-18 H-12 H-2 H-3 H-9

H-7, H3-18 H-12 H-2

H-14

H-14 H-3

H-9

Figure 1. Selected NOE correlations of 1.

spectrum of 2 (Table 1), signals for three acetate methyls were observed at δ 2.22 (3H, s), 2.11 (3H, s), and 1.94 (3H, s). The additional acyl group was found to be an n-butanoyl group, which showed seven contiguous protons (δ 0.95, 3H, t, J ) 7.5 Hz; 1.66, 2H, m; 2.26, 2H, t, J ) 7.5 Hz). The 13C NMR signal at δ 173.2 (s) correlated with the methylene protons at δ 2.26 in the HMBC spectrum and was consequently assigned as the carbon atom of the n-butyrate carbonyl. It was found that the NMR signals (1H and 13C) of 2 were similar to those of 1, except that the signals corresponding to an acetoxyl group in 1 were replaced by signals for an n-butyryloxyl group in 2. The n-butyrate ester was positioned at C-3 from the 1H-13C long-range correlation between H-3 (δ 4.89) and the carbonyl carbon of the n-butyrate (δ 173.2). The correlations from a NOESY experiment of 2 (Table 4) also showed that the stereochemistry of this metabolite is identical with that of 1. Thus, briaexcavatolide L (2) was found to be the 3-deacetyl-3-nbutyryl derivative of 1. Briaexcavatolide M (3) was obtained as a white powder. The molecular formula C24H31O9Cl was established by HRFABMS. The IR spectrum showed bands at 3433, 1780, and 1739 cm-1, consistent with the presence of hydroxyl, γ-lactone, and ester carbonyl groups. The FABMS of 3 showed peaks at m/z 501 [M + H + 2]+, 499 [M + H]+, 481 [M + H - H2O]+, 439 [M + H - HOAc]+, 379 [M + H 2HOAc]+, and 307 [M + H - 2H2O - 2HOAc - HCl]+, indicating the presence of two hydroxyl groups, two acetoxyl groups, and a chlorine atom in the molecule of 3. A

3

4

H-2, H3-15

H-2, H3-15

H-3, H3-15

H-3, H3-15

H-3, H2-16 H2-16 H2-16 H-6, H-9 H-7, H-9, H-10 H-10 H-9, H-14, H3-15

H-3, H2-16 H2-16 H2-16 H-6, H-9 H-7, H-9, H-10 H-10 H-9, H-14, H3-15

H-10, H-13 H3-20 H3-15

H-10, H-13 H3-20 H3-15

H-10

H-10

H3-18 H-17 H-17, H3-18 H-10

H3-18 H-17 H-17, H3-18 H-10

H-9 H-12

H-9 H-12

strong UV absorption at 234 nm suggested the presence of a conjugated diene system in 3. In the 13C NMR of 3 (Table 2), four olefin carbons were deduced from the signals at δ 140.8 (s), 136.8 (d), 126.2 (d), and 124.8 (d). A 13,14epoxide group was confirmed from the two tertiary oxygenated carbons at δ 62.4 (d) and 57.3 (d) and from the chemical shifts of H-13 and H-14 (δ 3.23, 2H, br s). From the 13C NMR spectrum of 3, three carbonyl resonances appeared at δ 175.7 (s), 170.5 (s), and 169.8 (s) and confirmed the presence of a γ-lactone and two other ester groups. In the 1H NMR of 3 (Table 1), two acetate methyls (δ 2.19, 3H, s; 2.13, 3H, s) also were observed. From an 1H-1H COSY experiment of 3 (Figure 3), it was possible to establish the separate spin systems that map out the proton sequences from H-2 to H-4, H-6 to H-7, H-9 to H-12, H-11 to H3-20, H-13 to H-14, and H-17 to H3-18. The HMBC data for 3 (Figure 3 and Table 3) revealed that two acetoxyl groups are attached to the C-9 and C-14 positions, respectively. Thus, the remaining hydroxyl group had to be positioned at C-8, an oxygen-bearing quarternary carbon resonating at δ 83.1 ppm. The relative stereochemistry of 3 was deduced from a NOESY experiment (Figure 4 and Table 4). The NOE correlations of H-10 with H-11 and H-2 indicated that these protons are situated on the same face and were assigned as the R protons since the C-15 methyl is β-orientated and H3-15 did not show correlation with H-10. The epoxy protons, H-13 and H-14 (δ 3.23), were found to exhibit NOE responses with H3-15, but not with H-10, revealing the β-orientations of these two protons. Furthermore, H-12 correlated with H-11, but not with the epoxy protons, indicating that the acetoxyl group at C-12 should be positioned on the β face. Additionally, it was found that H-7, H-9, and H-17 showed NOE correlations with each other. Consideration of molecular models revealed that H-9 is reasonably close to H-7 and H-17 when H-7 and H-17 are β-oriented, and H-9 is placed on the R face. On the basis

Briaexcavatolides K-N from Briareum

Journal of Natural Products, 2001, Vol. 64, No. 3 321

Table 4. Key NOESY Correlations for Diterpenes 1-4 H-2 H-3 H-4 H-4β H-6 H-7 H-9 H-10 H-11 H-12 H-13 H-13R H-13β H-14 H3-15 H2-16 H3-16 H-17 H3-18 H3-20

1

2

H-3, H-13R H-2, H-10, H-11

H-3, H-13R H-2, H-10, H-11

H-7, H-10 H3-16 H-4β, H3-15 H3-20 H-3, H-4β, H-11 H-3, H-10, H-12 H-11, H-13R

H-7, H-10 H3-16 H-4β, H3-15 H3-20 H-3, H-4β, H-11 H-3, H-10, H-12 H-11, H-13R

H-2, H-12 H-14 H-13β, H3-15 H-7, H-14

H-2, H-12 H-14 H-13β, H3-15 H-7, H-14

H-6

H-6

H-9

H-9

3

4

H-10 H-4, H3-15 H-3

H-10 H-4, H3-15 H-3

H2-16 H-9, H-17 H-7, H-17, H3-20 H-2, H-11 H-10, H-12, H3-18 H-11 H-14

H2-16 H-9, H-17 H-7, H-17, H3-20 H-2, H-11 H-10, H-12, H3-18 H-11 H-14

H-13, H3-15 H-14, H3-20 H-6

H-13, H3-15 H-14, H3-20 H-6

H-7, H-9 H-11 H-9, H3-15

H-7, H-9 H-11 H-9, H3-15

Figure 4. Selected NOE correlations of 3.

Figure 2. Computer-generated ORTEP of diterpene 1 showing relative configuration. Hydrogen atoms have been omitted for clarity.

Figure 3.

1H-1H

COSY and HMBC correlations for 3.

of the above findings, the molecular structure, including the relative stereochemistry of 3, was fully determined. Briaexcavatolide N (4) has the molecular formula C24H32O10, as determined by HRFABMS. The FABMS of 4 showed peaks at m/z 481 [M + H]+, 463 [M + H - H2O]+, 445 [M + H - 2H2O]+, 403 [M + H - H2O - HOAc]+, 385 [M + H - 2H2O - HOAc]+, 343 [M + H - H2O - 2HOAc]+, 325 [M + H - 2H2O - 2HOAc]+, and 307 [M + H - 3H2O - 2HOAc]+, suggesting the presence of three hydroxyl and

two acetoxyl groups in the molecule of 4. It was found that the 1H and 13C NMR spectral data of 4 (Tables 1 and 2) were very similar to those of 3. However, the NMR chemical shifts for H2-16 and C-16 of 4 (δH 4.30, 2H, br s; δC 63.8, t) were found to be shifted downfield, in comparison with the analogous data of 3 (δH 4.21, 2H, br s; δC 46.5, t), suggesting that the 16-chloro group of 3 was replaced by a hydroxyl group in diterpene 4. The relative configuration of 4 was also confirmed by a NOESY experiment (Table 4). Thus, the structure of 4 was clearly elucidated. Diterpene 5 was identified as a diterpene, (1S*,2S*,5Z, 7S*,8S*,9S*,10S*,11R*,12R*,13Z,17R*)-2,12-diacetoxy-8,17-epoxy-9-hydroxybriara-5,13-dien-18-one, which had been isolated from an Australian gorgonian. Its physical (optical rotation) and spectral (IR, MS, 1H and 13C NMR) data are in full agreement with those previously reported.6 Previous studies revealed that most chloro-containing briarane diterpenes have the chlorine atom located at C-6.7-13 The allylic chloromethyl group in 3, however, may come from the allylic rearrangement of a previously isolated compound, briaexcavatolide H (6).5 Also, it is worthwhile to note that briarane diterpenes containing both hydroxyl groups at the β face of C-8 and the R face of C-17 in the lactone ring, like 1 and 2, were observed for the first time. As it was observed that excavatolides C (7),2 B (8),2 and briaexcavatolide H (6)5 were stable and could not be converted into 1, 2, and 3, respectively, in solvents such as EtOAc, CHCl3, and EtOAc-hexanes (1:1) at room temperature, and because treatment of 7 with silica gel in EtOAc or with a trace amount of TFA in acetone also could not yield 1, the possibilities that 1-4 are artifacts arising from the corresponding hydrolysis of 7 and 8 and the allylic rearrangement of 6 during the extraction and the purification process could be ruled out. Thus, compounds 1-4 are

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regarded as the natural products of B. excavatum.

The cytotoxicity of the metabolites 1-5 against the growth of P-388 (mouse lymphocytic leukemia), A549 (human lung adenocarcinoma), and HT-29 (human colon adenocarcinoma) tumor cells was studied, and the results showed that the diterpenes 1, 3, and 4 were not cytotoxic toward the above cells. Compound 2 exhibited significant cytotoxicity against P-388 tumor cells with an ED50 of 0.5 µg/mL, and compound 5 exhibited significant cytotoxicity against P-388 and HT-29 tumor cells with ED50’s of 0.4 and 1.1 µg/mL.14 Experimental Section General Experimental Procedures. Melting points were determined using a Fisher-Johns melting point apparatus and were uncorrected. Optical rotations were measured on a Jasco DIP-370 digital polarimeter. IR spectra were recorded on a Hitachi I-2001 infrared spectrophotometer. UV spectra (in MeOH) were recorded on a Hitachi U-3210 UV spectrophotometer. FABMS were obtained with a VG Quattro GC/MS spectrometer. HRFABMS were recorded on JEOL JMS SX/ SX 102A mass spectrometer. The NMR spectra were recorded on a Varian VXR-300/5 FT-NMR at 300 MHz for 1H and 75 MHz for 13C or on a Bruker AMX-400 FT-NMR at 400 MHz for 1H and 100 MHz for 13C or on a Varian Unity INOVA 500 FT-NMR at 500 MHz for 1H and 125 MHz for 13C, respectively, in CDCl3 using TMS as an internal standard. Si gel (Merck, 230-400 mesh) was used for column chromatography. Precoated Si gel plates (Merck, Kieselgel 60 F-254, 0.2 mm) were used for analytical TLC. Animal Material. The gorgonian B. excavatum was collected by hand using scuba at South Bay, Kenting, located in the southernmost tip of Taiwan, in July 1995, at depths of 4-5 m, and was stored in a freezer until extraction. A voucher specimen was deposited in the Department of Marine Resources, National Sun Yat-Sen University (specimen no. KTSC-103). Extraction and Isolation. The extraction scheme followed the standard procedures of our group.2,3 The freeze-dried animal material (1.9 kg) was minced and extracted exhaustively with EtOAc. The EtOAc extract was chromatographed on Si gel column chromatography, using hexanes and hexanes-EtOAc mixtures of increasing polarity. Compound 5 was

Sung et al.

eluted with hexanes-EtOAc (4:1), compound 3 with hexanesEtOAc (2:1), compound 4 with hexanes-EtOAc (3:2), compound 2 with hexanes-EtOAc (3:2-1:1), and compound 1 with hexanes-EtOAc (1:1). Briaexcavatolide K (1): colorless prisms (26 mg); mp 270-273 °C; [R]27D -25° (c 1.0, CHCl3); IR (KBr) νmax 3428, 1772, and 1734 cm-1; 1H and 13C NMR data, see Tables 1 and 2; FABMS m/z 585 [0.2, (M + H)+], 567 (0.6), 525 (0.6), 507 (0.3), 465 (0.3), 447 (0.3), 405 (0.5), 345 (0.8), 327 (0.8), 309 (1), and 291 (2); HRFABMS m/z 585.2541 (calcd for C28H41O13, 585.2535). Briaexcavatolide L (2): white powder (22 mg); mp 164166 °C; [R]27D -37° (c 0.8, CHCl3); IR (KBr) νmax 3430, 1778, and 1730 cm-1; 1H and 13C NMR data, see Tables 1 and 2; EIMS m/z 552 [0.1 (M - AcOH)+], 534 (0.2), 524 (0.2), 506 (0.3), 492 (0.3), 464 (0.4), 446 (0.3), 404 (0.4), 386 (0.3), 344 (0.3), 326 (0.3), and 290 (0.3); HREIMS m/z 552.2554 [calcd for C28H40O11, (M - AcOH)+, 552.2559]. Briaexcavatolide M (3): white powder (6 mg); mp 211213 °C; [R]26D -29° (c 0.1, CHCl3); UV (MeOH) λmax 234 (log  ) 3.97); IR (KBr) νmax 3433, 1780, and 1739 cm-1. 1H and 13C NMR data, see Tables 1 and 2; FABMS m/z 501 [0.1, (M + H + 2)+], 499 [0.3, (M + H)+], 481 (0.3), 439 (0.3), 379 (0.3), and 307 (12); HRFABMS m/z 499.1740 (calcd for C24H32O9Cl, 499.1726). Briaexcavatolide N (4): white powder (7 mg); mp 174175 °C; [R]26D -23° (c 0.2 CHCl3); UV (MeOH) λmax 236 (log  ) 3.94); IR (KBr) νmax 3421, 1770, and 1733 cm-1; 1H and 13C NMR data, see Tables 1 and 2; FABMS m/z 481 [0.5, (M + H)+], 463 (0.8), 445 (0.6), 403 (0.8), 385 (0.6), 343 (0.6), 325 (0.7), and 307 (10); HRFABMS m/z 481.2071 (calcd for C24H33O10, 481.2064). Diterpene 5: white powder (10.9 mg); mp 67-68 °C; [R]25D -42° (c 0.3, CHCl3) (lit.6 [R]D -44.9° (c 0.31)); IR (KBr) νmax 3488, 1784, and 1728 cm-1; 1H NMR (500 MHz, CDCl3) δ 5.87 (1H, dd, J ) 10.0, 6.0 Hz, H-13), 5.65 (1H, d, J ) 8.5 Hz, H-7), 5.52 (1H, d, J ) 10.0 Hz, H-14), 5.32 (1H, d, J ) 8.5 Hz, H-6), 4.91 (1H, dd, J ) 6.0, 2.0 Hz, H-12), 4.85 (1H, d, J ) 8.5 Hz, H-2), 3.73 (1H, dd, J ) 10.0, 5.0 Hz, H-9), 3.20 (1H, dd, J ) 10.0, 4.0 Hz, H-10), 2.58 (1H, m, H-3β), 2.55 (1H, dd, J ) 12.0, 3.5 Hz, H-4β), 2.30 (1H, m, H-11), 2.23 (3H, s, acetate methyl), 2.07 (3H, s, acetate methyl), 1.93 (1H, br t, J ) 12.0 Hz, H-4R), 1.86 (3H, s, H3-16), 1.68 (1H, m, H-3R), 1.59 (3H, s, H3-18), 1.21 (3H, s, H3-15), and 0.98 (3H, d, J ) 7.5 Hz, H3-20); 13C NMR (125 MHz, CDCl3) δ 172.4 (s, C-19), 170.4 (s, acetate carbonyl), 168.0 (s, acetate carbonyl), 142.1 (s, C-5), 140.3 (d, CH-14), 123.4 (d, CH-13), 121.0 (d, CH-6), 82.0 (d, CH-2), 73.7 (d, CH-7), 70.7 (s, C-8), 70.2 (d, CH-12), 68.4 (d, CH-9), 58.7 (s, C-17), 43.2 (s, C-1), 36.3 (d, CH-10), 32.6 (d, CH-11), 24.4 (t, CH2-4), 23.9 (q, CH3-16), 23.0 (t, CH2-3), 21.3 (q, acetate methyl), 20.8 (q, acetate methyl), 19.1 (q, CH3-15), 13.6 (q, CH320), 9.4 (q, CH3-18); FABMS m/z 449 [0.6, (M + H)+], 389 (1), 329 (2), and 311 (2). The physical and spectral data of 5 are in full agreement with those reported previously.6 Single-Crystal X-ray Crystallography of Briaexcavatolide K (1).15 Suitable colorless prisms of 1 were obtained from a solution in EtOH. The crystal (0.50 × 0.56 × 0.60 mm) belongs to the monoclinic system, space group P212121 with a ) 8.568(3) Å, b ) 17.490(4) Å, c ) 19.846(2) Å, V ) 2974(1) Å3, Z ) 4, Dcalcd ) 1.306 g/cm3, λ(Mo KR) ) 0.71069 Å. Intensity data were measured on Rigaku AFC7S diffractometer up to 2θ of 50.0°. All 3000 unique reflections were collected. The structure was solved by direct methods and refined by a fullmatrix least-squares procedure. The non-hydrogen atoms were given anisotropic thermal parameters. The refinement converged to a final R ) 0.044, Rw ) 0.033 for 1987 observed reflections [I > 3.00σ(I)] and 370 variable parameters. Cytotoxicity Testing. The P-388 cell line was kindly provided by Prof. J. M. Pezzuto, University of Illinois at Chicago; A549 and HT-29 cells were purchased from the American Type Culture Collection. The cytotoxicity of tested compounds 1-5 against the above three cancer cells was assayed with a modification of the MTT [3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide] colorimetric method.16

Briaexcavatolides K-N from Briareum

Acknowledgment. This work was supported by a grant from the National Science Council of the Republic of China (NSC-89-2113-M-110-007) awarded to J.-H.S. A fellowship (to P.-J.S.) from the National Science Council of the Republic of China is also acknowledged. References and Notes (1) Sheu, J.-H.; Sung, P.-J.; Su, J.-H.; Wang, G.-H.; Duh, C.-Y.; Shen, Y.-C.; Chiang, M. Y.; Chen, I.-T. J. Nat. Prod. 1999, 62, 1415-1420, and references therein. (2) Sheu, J.-H.; Sung, P.-J.; Cheng, M.-C.; Liu, H.-Y.; Fang, L.-S.; Duh, C.-Y.; Chiang, M. Y. J. Nat. Prod. 1998, 61, 602-608. (3) Sung, P.-J.; Su, J.-H.; Wang, G.-H.; Lin, S.-F.; Duh, C.-Y.; Sheu, J.H. J. Nat. Prod. 1999, 62, 457-463. (4) Neve, J. E.; McCool, B. J.; Bowden, B. F. Aust. J. Chem. 1999, 52, 359-366. (5) Sheu, J.-H.; Sung, P.-J.; Su, J.-H.; Liu, H.-Y.; Duh, C.-Y.; Chiang, M. Y. Tetrahedron 1999, 55, 14555-14564. (6) Bowden, B. F.; Coll, J. C.; Vasilescu, I. M. Aust. J. Chem. 1989, 42, 1705-1726. (7) Burks, J. E.; van der Helm, D.; Chang, C. Y.; Ciereszko, L. S. Acta Crystallogr. 1977, B33, 704-709.

Journal of Natural Products, 2001, Vol. 64, No. 3 323 (8) Look, S. A.; Fenical, W.; Van Engen, D.; Clardy, J. J. Am. Chem. Soc. 1984, 106, 5026-5027. (9) Groweiss, A.; Look, S. A.; Fenical, W. J. Org. Chem. 1988, 53, 24012406. (10) Shin, J.; Park, M.; Fenical, W. Tetrahedron 1989, 45, 1633-1638. (11) Kobayashi, J.; Cheng, J.-F.; Nakamura, H.; Ohizumi, Y.; Tomotake, Y.; Matsuzaki, T.; Grace, K. J. S.; Jacobs, R. S.; Kato, Y.; Brinen, L. S.; Clardy, J. Experientia 1991, 47, 501-502. (12) Pordesimo, E. O.; Schmitz, F. J.; Ciereszko, L. S.; Hossain, M. B.; van der Helm, D. J. Org. Chem. 1991, 56, 2344-2357. (13) Maharaj, D.; Pascoe, K. O.; Tinto, W. F. J. Nat. Prod. 1999, 62, 313314. (14) For significant activity of pure compounds, an ED50 value of e 4.0 µg/mL is required. Geran, R. I.; Greenberg, N. H.; MacDonald, M. M.; Schumacher, A. M.; Abbott, B. J. Cancer Chemother. Rep. 1972, (3), 1-91. (15) Crystallographic data for this structure have been deposited with the Cambridge Crystallographic Data Centre. A copy of the data can be obtained, free of charge, on application to the Director, CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (fax: +44-(0)1223-336003 or e-mail: [email protected]). (16) Mosmann, T. J. Immunol. Methods 1983, 65, 55-63.

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